Replace memchr with manual memory search
Replace memchr with manual memory search

On large files with predominantly \n line endings, memchr(..'\r'..)
seems to waste a considerable amount of time searching for a line
ending candidate that we don't need.

On the Adobe PDF Reference Manual 1.7, this commit is 8x faster at
QPDF::processMemoryFile().

jberkenbilt/qpdf,qpdf/qpdf,qpdf/qpdf,jberkenbilt/qpdf,qpdf/qpdf,jberkenbilt/qpdf,qpdf/qpdf,jberkenbilt/qpdf,qpdf/qpdf,jberkenbilt/qpdf